Mechanical actuators are used in a variety of applications for controlling the operation of various mechanical components or devices in systems. The term actuator here is used to denote an apparatus which has an electrically powered drive mechanism providing mechanical power to an output element which applies the mechanical force for operating the component. The output element is continuously movable toward either of first and second extreme positions responsive to electrical actuator power of first and second types respectively supplied to the drive mechanism. These actuators have a power supply of some type which provides electrical power across first and second power terminals. These actuators also have a control circuit or controller receiving electrical power from the power supply's power terminals for selectively converting electrical power from the power supply responsive to an externally provided control signal, into electrical actuator power of the first and second types. The controller supplies to the drive mechanism the type of electrical actuator power which drives the output element in the direction desired.
The description following is intended to address both linear and rotary actuators, and the invention can easily be applied to both. Those familiar with the principles of kinetics understand that it is easy to transform linear force into torque and torque into linear force by the use of a rack and pinion, a cam and follower, or a crank arm. In a typical actuator, the drive mechanism includes an electric motor and a gear train of some kind for multiplying the force or torque available from the motor which is to be supplied as the actuator's output. The output element of rotary actuators will typically have a maximum rotation of less than one complete revolution. Rotary actuators now available for controlling devices such as valves and air dampers use small electric motors which drive through speed reduction gear trains having a ratio of a thousand to one or more. Thus, to move the actuator output element from one extreme excursion to the other, a motor having a shaft speed of a few thousand RPM will typically have to run for a few tens of seconds.
There are many different types of systems which employ these actuators. For example, burner systems have fuel valves and air supply dampers for controlling the flow of these fluids to the burner's combustion chamber. Actuators are customarily used to operate these components. Chemical plant systems frequently have large numbers of actuator-operated valves for controlling flow of fluids by which the processes of these plants are implemented. In most of these systems, a system controller prescribes a sequence of operation for the system components including the actuators. The controller commands the actuators to operate the components which they control at selected times. If the sequence and timing of operation for actuators is not properly executed, there may well be safety and quality implications for the system. For example, it is well known that if the fuel control valves of a burner system are not closed and opened at the proper times, potentially dangerous conditions may result.
In many systems, the actuators each have a setting or position which provides for safety or prevents damage should the operating sequence be interrupted. One example might be in burners, where if the controller fails, it is important to immediately close the fuel valves. A frequent cause of controller failure is loss of power for the system. Such situations may arise because of a generalized blackout involving the electrical mains, a tripped circuit breaker, or a failure of the actuator's power supply. This situation has particular implications for electrically powered actuators since once electrical power is lost, electrical power is no longer available to operate the components which they control. In the past this situation has been addressed by providing an alternative energy store such as a spring or battery which provides power for returning the component to its safety position when power to the actuator is lost. Newer actuator designs use a capacitor instead of a spring to store energy for returning the controlled component to its safety position when power is lost.
Capacitors have been used for interim power supplies in a variety of systems for a long time. One common example is the use of a capacitor to provide bridge power to the station frequency memory during battery replacement in small portable radios of the type having synthesized tuning. This avoids the need each time the batteries are replaced, for the user to reprogram the possible 10 or 15 station frequencies previously recorded in the memory. There are also actuators now described in the literature which use a capacitor to provide interim power in the case of a power outage. These actuators use a separate charging circuit for the capacitor, and an automatically operated power switch to select either the normal power supply or in the case of a power outage, the capacitor as the source for the motor current.